1. Field of the Invention
The present invention relates to a pyrotechnic mixture in the form of granulates, encompassing .a nitrogen-generating substance, an inorganic oxidizing medium and a binder constituted of silicone rubber. Moreover, the invention relates to a process for the production of this pyrotechnic mixture, as well as to a gas generator for an airbag which contains the pyrotechnic mixture as a propellent charge.
2. Discussion of the Prior Art
From the disclosure of German DE A 39 20 401 there is presently known a gas generator for an airbag, in which a chamber is positioned about a cylindrical pressure container which is filled with gas-liberating substances. This chamber encompasses a filter. This filter receives an oncoming flow of the gas through apertures which are arranged at a base end thereof. The gas then streams through the filter in the longitudinal direction of the latter and passes through apertures provided at the head end of the chamber into the airbag. That type of gas generator, in view of the presence of its pressure container, necessitates the provision of sufficiently heavy wall thicknesses. In addition thereto, it is necessary to provide a separate chamber for the installation of the filter. As a consequence, the gas generator becomes expensive. Considerable flow losses which are encountered because of the presence of the filter lead to lower mass flows; in essence, meaning that the period of time which elapses commencing from the ignition of the gas-liberating substances until the gas has filled the airbag is lengthy. The propellent charge is present in the form of tablets.
Further gas generators for airbags are described; for example, in U.S. Pat. Nos. 3,907,330, 5,087,070 and 5,149,129. Also in such gas generators is the propellent medium ordinarily employed in the form of pellets, tablets or cylindrical molded bodies. In order to break open these molded bodies, it is necessary to expend a relatively considerable quantity of energy. Further energy losses are encountered through the lengthy path of flow for the gases in the gas generator.
European Patent A-0 467 731 pertains to a solid pyrotechnic composition, which contains an alkali metal or alkaline earth metal azide, an inorganic oxidizing medium and, as a component in the amount of 5%-40% of the total composition, an inorganic binder medium, which is the product of a polycondensation reaction of alumino silicates. Pursuant to EP-A-0 467 731 (page 2, lines 42 through 45), organic binder media in pyrotechnic compositions are not considered to be particularly suitable, inasmuch as they propagate the formation of toxic combustion gases.
German Patent Publications DE-A-37 27 851 and DE-A-37 44 750 relate to pyrotechnic compositions in the form of lumpy pieces or bodies which contain fibers in a proportion of 2%-6% by weight, generally such as graphite fibers, or fibers consisting of iron or glass fibers. In order to improve upon the ignition, the lumpy body can be imparted a coating which; among other constituents, contains silica, a fluorine elastomer and magnesium. The fibers serve for reinforcing the lumpy body or piece and minimize the possibility in the development of fissures in the body through which there are created undesirable additional surface areas which can conceivably produce an acceleration in the speed of combustion of the lumpy body in an unpredictable manner.
The Japanese Patent Publication 63-57258 relates to a pyrotechnic composition for airbags, which contains a buffer material in the form of a mesh of stainless steel fibers. For an airbag on the driver's side (which usually possesses a volume of about 60 liters), there are employed 87 grams of the gas-generating mixture. For an airbag on the passenger side (which usually possesses a volume of about 120 to 150 liters), there are employed 400 grams of the gas-generating mixture. These large quantities of gas-generating mixture lead to disposal problems; for instance, to an extremely high degree of heating during the burning off of the mixture, whereby this heat must be contained through the construction of the gas generator in order to prevent a bursting of the airbag.
German OS 23 34 063 and German OS 23 51 379 describe the utilization of silicone rubber as a nitrogen-free oxygen-containing binder medium in a composite powder which contains 78% to 92% by weight of a chlorate or perchlorate oxidizing medium. The preferred content of the silicone rubber is indicated as being 8% to 14.6% by weight. Beyond this limit of the content of the binder medium there is attained an excessively high carbon monoxide content during the combustion. The composite powder is compressed into compacts or bars each having a length of 30 mm and a diameter of 4.3 mm (referring to German OS 23 51 379, page 8).
The published Japanese Patent Appln. 50-040487 describes the addition of SiO.sub.2 -containing inorganic binder media to pyrotechnic mixtures, which contain metal azide and an inorganic oxidizing medium. It is reported that, for a constituent of more than 10% of a binder medium, the ignition of the pyrotechnic composition is considerably slowed down, whereby such a content is not suitable for use in actual practice.
German DE-A-41 16 879, published on Nov. 26, 1992, describes a gas-generating mixture in the form of granulates, which consist of a metal azide, an oxidizing medium and nitrocellulose, and which are coated with a rapidly burning igniting mixture which contains boron and potassium nitrate. This granulated gas composition is provided with silicone rubber in a proportion of 35% relative to the gas-generating mass, and formed into a foil-shaped elastic layer. The employment of a mixture which is based on a metal azide, and an oxidizing medium and silicone rubber in the form of granulates as a propellent charge in airbags is not taught in this publication.
A disadvantage of pyrotechnic mixtures for airbags which are known from the current state-of-the-technology consists of in that, in order to achieve a high packing density and to avoid dissociation of the ingredients, these must be employed in the form of tablets, cylindrical bodies or foils which, due to the small available-reaction surface, leads to that large quantities of these mixtures are required for the inflating of the airbag. Furthermore, during the burning down of the pyrotechnic mixtures, there are produced large quantities of strongly alkaloid substances (for example, K.sub.2 O, Na.sub.2 O), which notwithstanding the employment of inorganic binder media which are based on silicates, can only be poorly neutralized.